Add alertmanager to update-permissions
Fix permission for prometheus's alertmanager
folder to prevent the following error after Docker migration:
2018-08-10_09:30:22.93310 level=error ts=2018-08-10T09:30:22.933055374Z caller=main.go:226 err="open /var/opt/gitlab/alertmanager/data/nflog: permission denied"
2018-08-10_09:30:23.96010 level=error ts=2018-08-10T09:30:23.960038191Z caller=main.go:226 err="open /var/opt/gitlab/alertmanager/data/nflog: permission denied"
2018-08-10_09:30:24.98210 level=error ts=2018-08-10T09:30:24.982043183Z caller=main.go:226 err="open /var/opt/gitlab/alertmanager/data/nflog: permission denied"